1. Field of the Invention
The present invention relates to method to fabricate a protector device and the device fabricated by the method.
2. Related Background Art
A protector device including a lightning discharger plays a very important role in protecting various electronic devices from surge including lightning. A protector device is a general name of devices which are used in order to protect other electronic devices from excess voltage, that is surge. An arrester is used to protect other electronic devices from lightning, that is extremely high voltage and large current. An arrester is one of the protector devices. The term of a protector device is used here to indicate devices which are used in order to protect other electronic devices from excess voltage. However, excess voltage is not limited to extremely high voltage but also includes a low voltage exceeding a specified voltage.
A glass tube type arrester has been used. It contains special gas between two electrodes in a glass tube. It is non-conductive unless surge is induced. When surge or lightning is induced, discharge starts and the gas between the electrodes changes to conductive. Current flows through the arrester and is led to the earth. Discharge does not stop immediately after surge is removed. The arrester cannot protect other electronic devices from continuous current or a next attack by surge or lightning. There are serious problems which a glass-tube and other type protector devices have. One of them is that a protector device must change from its resistive state to a conductive state in a very short time such as 0.03 xcexcsec. when it is attacked by surge. Another problem is that a protector device should return from its conductive state to its resistive state when surge is removed.
In order to overcome the problems an arrester was proposed (Japanese Patent 118361, 1995 xe2x80x9cMolybdenum arresterxe2x80x9d by Seita Ohmori). It used plural molybdenum bars whose surface was oxidized. The arrester will be referred to as a xe2x80x9cmolybdenum arrester.xe2x80x9d
The molybdenum arrester leads current to the earth in a very short time when surge or lightning is induced. That is, it changes from non-conductive state to conductive state very quickly by breakdown of the oxide formed on the molybdenum bar. Moreover, it returns from conductive state to non-conductive state when surge or lightning is removed because molybdenum is oxidized quickly if it is in oxidizing atmosphere. The molybdenum arrester is very useful and economically efficient because it repeats change of the state automatically.
It is possible to use other metals than molybdenum in a protector device which functions with the same principle as the molybdenum arrester. Tantalium, chromium and aluminum are included in such metals. The principle of the molybdenum arrester can be applied also to a device in which single bar is used.
Although the molybdenum arrester has superior properties than other type arresters, it has been difficult to fabricate by an automation process. The reason was that delicate control was necessary during the fabrication. That is, it was necessary to control force applied to the interface between resistive films in the molybdenum arrester because a breakdown voltage at the interface depends on the force. In addition, electrical resistance at the interface between the molybdenum bars having a resistive film on it and the conductive part of the case or cap depends also on the force applied to the interface. The interface was used as an electrical contact between the molybdenum bar and the case or the cap. The reason why mechanical contact between the molybdenum bar and the case or the cap was used as an electrical contact was that it is possible that an electrical contact formed by alloying melts by heat produced by large current induced by surge. Electrical resistance of the contact decreases with increase of applied force. However, larger force does not give the optimum breakdown voltage at the interface between the resistive films. It was necessary, therefore, to control during fabrication force applied between the top of the cap and the bottom of the case in which the molybdenum bars are included. Control of the force by machine has been difficult.
The present invention is directed to useful methods to fabricate surge protector devices including arresters. The methods of the present invention make it possible to fabricate surge protector devices by automation process. In particular, in the fabrication process according to the present invention, the forces applied to the interface between the cap and the case and the metal bars, and the interface between the resistive films are controlled automatically.
In the method of the present invention, a case in which metal bars are fixed and oxidizing and refractory agents are put in, a cap, metal bars with a resistive film on the surface, oxidizing and refractory agents are prepared in advance. At least a part of the bottom of the case and at least a part of the top of the cap are made conductive and form electrical contacts to the metal bars.
The case and the cap can be joined to form a single body which can be sealed at the late stage of the process.
The fabrication process according to the present invention includes following steps.
At the first step, the case is fixed on the holding table. The bottom of the cap has a shape adapted to fix the metal bar.
At the next step, the metal bars are inserted into the case such that at least a part of the lowest metal bar contacts with the inner side of the conductive part of the bottom of the case. Plural metal bars are piled up upwards.
At the next step, the oxidizing and the refractory agents are put in the case. The case is then vibrated in order to stabilize the oxidation and the refractory agents. The metal bars are fixed to the case during the step to give vibration in order not to change relative position between the bar and the case. The oxidizing and the refractory agents are added if necessary and vibration is given again. The steps are repeated until a predetermined amount of the oxidizing and the refractory agents are put in the case. The oxidizing and the refractory agents are put in such that all metal bars except the uppermost metal bar are buried in the agents.
At the next step, the cap is put on the case such that at least a part of the inner side of the conductive part of the cap contacts at least a part of the uppermost part of the uppermost metal bar.
At the next step, a predetermined force is applied between the cap and the case, and they are fixed to a single body keeping the force.
At the next step, the body is sealed.
In another embodiment, the fourth step can be done by other methods. For example, in third embodiment divided covers are put on the case such that there is no gap between the vacuum gripper and the covers as shown in FIG. 15. At least one of the divided covers has an inlet port through which the oxidizing and the refractory agents can be inserted. The oxidizing and the refractory agents are inserted with pressurized air into the case while the covers are pressed so that they are not lifted. The vacuum gripper holds the metal bar tightly.
Further in other embodiment, it is possible to put an inner cap on the case while the vacuum gripper holds the metal bar. An inner cap fixes a part of the metal bar. After the inner cap in fixed, the vacuum gripper is removed from the metal bar. Then an outer cap is put on the inner cap and pressed with a predetermined force followed by fixing.